Repeater system for strong signal environments

ABSTRACT

A repeater system combines co-located antennas, an intentional imbalance in signal amplification between downlink and uplink, stability management and an amplification indicator to create a user-installed solution to co-channel interference within cellular systems, in strong signal environments such as elevated locations or high-rise building. The invention may be particularly relevant to cellular systems, such as CDMA, that allow limited imbalance between uplink and downlink path losses, thus enabling the design of an inexpensive repeater with a weaker or non-existent uplink, and which creates moderate signal amplification for selected line-of-sight signals, defeating co-channel interference over a small area. The difference in signal amplification on the downlink and uplink is maintained at a level below the capacity of the system to support imbalance, guaranteeing reliable cellular calls.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 11/289,877, filed Nov. 30, 2005, which is acontinuation-in-part of U.S. patent application Ser. No. 10/794,458,filed Mar. 4, 2004, which claims priority from U.S. ProvisionalApplication Ser. No. 60/451,397, filed Mar. 4, 2003, the contents ofwhich are herein incorporated by reference in their entireties.

FIELD

This present invention relates generally to repeater systems for mobilecommunication systems such as cellular telephone systems, and moreparticularly to imbalanced repeater systems.

BACKGROUND

As is well known, cellular and PCS systems provide two-way audio anddata communications to subscribers, deploying hundreds of cell sites ina typical large city to create coverage over 95% or more of the targetedarea. Downlink signals are transmitted to cellular subscriber telephonesfrom directional base station antennas mounted at 30-100 ft above groundlevel. Uplink signals are received from subscribers by the samedirectional antennas.

In the United States in 2003, there were 127M subscribers to thecellular service providers available in each urban market. World marketsize was about 800M in 2003. Subscribers with telephones using CDMAtechnologies represent 44% of the U.S. market, while subscribers withtelephones using TDMA, GSM, and AMPS technologies represent the other56%.

Power control algorithms in the cellular network establish and vary theuplink power levels to be transmitted from the subscriber unit (cellphone) in order to maintain good call quality and to minimizeinterference to other calls. Downlink power levels are either static, orare varied to a lesser degree, relying on information from thesubscriber unit in order to determine what downlink power levels willgive good audio quality. Measurements of uplink signal quality areperformed by the base stations and then power control commands areissued to the subscriber units to maintain a minimum or ideal signalquality. Cellular systems are designed for a “balanced link” so that theuplink and downlink cell radii are the same, and so that uplink anddownlink handoff boundaries are coincident. Balanced in this sense maymean less than 1 dB of difference between the two directions.

CDMA, or code division multiple access, cellular systems are defined byIS-95, J-Std-008, and the evolving UMTS (Universal Mobile TelephoneSystem) standards. In CDMA systems, subscriber unit transmit power isinitially based on the received downlink power, received from a fixedpilot power level that all base stations transmit. The subscriber unituses its received power, and the knowledge that, on average, the pathloss is the same for uplink and downlink (balanced), to calculate anappropriate uplink power to transmit with in order to meet the samesignal quality requirement as that used on the downlink. Once call setuphas begun, the uplink receiver at the base station takes over subscriberpower control by transmitting messages to the subscriber, incrementingpower up or down 800 times per second to maintain a target signalquality level. Uplink power control then is substantially independentfrom the received downlink signal level once call setup begins, and canoffset the link balance up to a programmed amount.

Imbalance in path loss between the downlink direction and the uplinkdirection occurs when a phenomenon known as fast-fading (same assmall-scale fading) occurs independently on the uplink and downlink,leading to so called “opposite fading.” Fast fades can represent dropsin average power, every half-wavelength or so, of 20 dB or more. So, theuplink and downlink can be temporarily offset by 20 dB or more at times.Longer-term imbalance can also occur due to system calibration errors,due to noise rise fluctuations at the base station receiver, and due tovariations in diversity antenna gain.

Typical causes of poor call quality include insufficient capacity, weakcoverage, and strong interference. Capacity is the ability to handlemany calls (e.g., a lack of capacity results in a blocked call).Capacity can be increased by re-using the frequencies allocated to thatservice provider many times over in a single city. TDMA, GSM, and AMPSsystems use seven-cell reuse patterns, meaning adjacent cells usedifferent frequency channels and/or time slots to prevent co-channelinterference. CDMA uses a one-cell reuse pattern, meaning every celluses the same frequency channel all of the time. In this case, talkchannels are separated by coding.

Coverage holes sometimes occur in valleys, tunnels, buildings, and inplaces where there are no nearby base stations. The coverage hole in abuilding is either the central area of a floor, away from the windows,or the entire floor. Generally, the upper floors of tall buildings inurban areas have very strong signals from several LOS or near-LOS basestations. Under LOS conditions, path loss behaves approximatelyaccording to d² (where d is the one-way distance between the antennasystem of the base station and the antenna of the subscriber unit),which means losses increase 4× (or 6 dB) for each doubling of distancebetween the base station and the subscriber unit. Under LOS conditions,the subscriber can potentially see the base station. Under near-LOSconditions, there may be additional losses, such as those caused bydiffraction, which bends the rays coming from the base station as theypass by the edge of an obstruction. In LOS and near-LOS conditions, mostof the energy arriving at the subscriber unit occurs within a narrowangular spread from one general direction.

The cellular concept works because of terrestrial propagation, providingisolation between cells using the same frequency (co-channel cells) viamanmade clutter, trees, and terrain. For non-LOS signal paths, path lossbehaves approximately according to d⁴, meaning loss increases 16× (or 12dB) for each doubling of distance between cell site and subscriber unit.As long as a user is on or near ground level, the system will work asplanned and provide nearly interference-free performance withpredictable handoff boundaries. In urban areas for subscribers on theground, non-LOS conditions prevail, because of the interceding clutter,and the radio energy is scattered over a nearly 360° angular spread,arriving at the subscriber from many directions at once, summing at theomni-directional antenna.

One problem that occurs in strong-signal locations is co-channelinterference. Co-channel interference occurs when the signal receivedfrom two or more cell sites using the same frequency are adequate (>−90dBm) and comparable in signal strength, resulting in poor audio qualityor the inability to place or receive a phone call. This may occur, forinstance, on the upper floors (e.g., floor 6 and up) of high-risebuildings, such as apartments and offices, because of the breakdown ofthe terrestrial cellular concept and the occurrence of LOS and near-LOSconditions with several nearby base stations. When a subscriber unitlocated in such a location “sees” several co-channel cell sites, pooraudio quality or “no service” occurs for the user and the spectrumoperator experiences a reduction in billable airtime. All technologiesexperience co-channel interference on uplink and/or downlink. Strongsignals in high-rise buildings are typically in the range of −90 dBm to−50 dBm. Because of the strong signal levels, the subscriber unit iswell within the uplink and downlink range limits of the cell design.

CDMA is particularly vulnerable to this problem, as is anycommunications technology that has a small frequency reuse factor. CDMAco-channel interference is called pilot pollution. CDMA is moresusceptible to co-channel interference in elevated locations than othertechnologies because of one-cell reuse factor, instead of the seven-cellreuse used by TDMA, GSM, and AMPS systems. Once as many as four-to-sixpilot signals (cells) are received by a subscriber unit at approximatelythe same signal strength/quality, the telephone cannot lock onto asignal and it may be difficult to impossible to obtain service (callscannot be placed or received)

Even if service is obtained, the user experiencing pilot pollution mayhear a break-up in the audio signal as he/she moves about the room.Since the uplink power control in CDMA has a dynamic range of 80 dB andis managed well, pilot pollution is generally only a problem in thedownlink direction. While the presence of an uplink transmission from asubscriber unit in a high-rise building may have an effect on many CDMAbase stations, possibly decreasing capacity slightly, the power controlalgorithms keep all current phone calls equal in received power level sono one call is interfered with. It is estimated that ten to twenty-fivepercent of windowed rooms located on or above the sixth floor have pilotpollution.

There are several million high-rise office and apartment rooms in theU.S., and interference is usually the strongest nearer the window, wherethere is LOS visibility to several base stations. Generally, theinterference diminishes as the user moves away from the window and theassociated outage volume, and into the core of the building. This isbecause the building acts as a directional antenna, selectivelyattenuating some of the co-channel signals, resulting in lessinterference. Often, one side of a building will have the problem andthe other side will not. As a result, co-channel interference tends toconcentrate in a subset of the windowed rooms within an affectedbuilding, and only some individuals will require a solution. In othersituations, an entire floor may experience co-channel interference, andthere may be several or many residents who want to restore cellularservice.

Unfortunately, subscribers cannot distinguish, generally, between aninterference problem and a coverage problem. The subscriber justexperiences poor audio or no service. As a result, the only optionsavailable to subscribers are to complain to their provider and/or changeproviders (churn). Since there is incomplete feedback to the provider asto the nature of a customer's problem, the provider may haveinsufficient information to design a customer-specific solution. Twentymillion CDMA subscribers are expected to leave (churn) their U.S.provider in 2004 due to coverage, interference, or pricing (based on127M subs, average churn of 37% per year, and 44% CDMA).

Two-way personal repeaters and two-way higher-power indoor and outdoorrepeaters are “coverage repeaters,” designed to solve coverage problemsdue to weak signals in outdoor and indoor locations using balancedamplification of uplink and downlink. Balanced amplification of bothlinks maintains the “balanced link” design, which is important in a weaksignal condition since it is desirable to extend both uplink anddownlink cell radii equally into the weak signal area. Coveragerepeaters are occasionally applied to co-channel interference problems.Coverage repeaters are designed for larger areas, such as partialfloors, whole floors, or whole buildings, and are not economical forsmaller areas of interference (e.g. an apartment or office room).Furthermore, an indoor coverage repeater installation includes aremotely-mounted (not co-located) highly directional pickoff antenna(e.g. 30° beamwidth), often a Yagi, to pick-off a single base station(known as a donor cell). The pickoff antenna is usually placed at ahigher elevation (such as the roof of the building) than the area ofweak coverage in order to collect a strong and particular LOS signal,unavailable at the subscriber unit, and must be positioned/adjusted topoint at the desired donor cell. The signal gain experienced by thesubscriber is as dependent on the signal field at the pickoff antenna asit is on the amplifier gain and the antenna gains. When applyingcoverage repeaters to interference problems, a remote pickoff antenna isstill needed in order to establish the donor signal well above the noisefloor and above adjacent spectrum signals prior to amplification so thatthe indoor re-radiating antenna does not cause interference toother-system subscribers. The installation includes a coaxial run torelay the pickoff signal back to the repeater unit and indoorre-radiating antenna(s). The installation also includes a downlink anduplink amplifier chain and an uplink interference control mechanism, viaa control circuit and/or operator coordination/engineering, that setsgain appropriately in order to avoid interference to the larger outdoorsystem. The installation also includes setting downlink gain, eithermanually, or automatically, to match uplink gain and avoid oscillationdue to excessive antenna-antenna feedback. The installation furtherincludes a re-radiating antenna or a distributed antenna system. Often,a method for monitoring the repeater for malfunction is incorporated inthe installation in order to notify the operator of potentialinterference to same or other communications systems. For example, theoccurrence of oscillation in the repeater, occurring at some frequencywithin the pass band of the filtering circuits, may transmit aninterfering signal, at rated power, to one or more base stationreceivers, or to one or more subscribers. Oscillation within the uplinkstages of the repeater may interfere with the performance of donor cellsof the system intended to be enhanced by the repeater, or may interferewith the performance of base station receivers belonging to systems notserved by the repeater installation. In addition, oscillation within thedownlink stages of the repeater may interfere with subscribers that areserved by the repeater installation, or with subscribers on adjacent RFchannels or in adjacent spectra owned by other communications systems.

Personal (coverage) repeaters are lower power version of standardrepeaters, have lower gain (e.g. 50-60 dB), and are designed to serve asingle floor or partial floor. Personal repeaters have limited range,however, and if applied to solving pilot pollution in a windowed room,may not extend to the interior or core area of the same high-rise floor.

Coverage repeaters have the following disadvantages: they are costly,they require engineering, they pose a risk to the uplink performance ofthe same-spectrum and adjacent-spectrum cellular systems, and they areoptimized for large areas shared by many subscribers. Personal coveragerepeaters are expensive—$500 to $3000—compared to the cost of changingservice providers. A weatherproof outdoor antenna, remote mounting, ahighly-directional pickoff, controlling the uplink gain (circuit and/orengineering), installing a coaxial run, and system monitoring all addcost to a repeater installation. Coverage repeaters require complexinstallation because a donor site must be selected and a coaxial cablerun and roof/outdoor pickoff antenna mounting is required with a holethrough the roof or wall. They run the risk of system interference andrequire engineering and operator coordination. If oscillation occurs dueto changes in the path loss environment, generally a shutback circuitreduces gain or turns the amplifier off, disabling the repeater, whichthen requires a technician to re-optimize the gain setting or theinstallation. The solution is not cost effective for an individualexperiencing co-channel interference within an office or apartment sincecoverage repeaters are optimized to solve coverage (weak signal)problems. Many of the elements are intended to address other issues thana high-rise interference problem that may only be experienced by asingle user. These elements include a highly-directional antenna, uplinkgain, uplink interference control, remote pickoff antenna mounting andthe associated coaxial run, and repeater monitoring to protect thesystem from interference. Lastly, the pickoff signal strength isunpredictable (until a signal measurement is made at the pickofflocation), so the gain needed in the user ambient environment issomewhat unknown.

It is important to recognize that mobile telecommunication systems arealways designed to be balanced systems, in that the downlink path lossesare equal to the uplink path losses. This is done so that the cellboundaries (areas where hand-offs occur) are the same for both thedownlink and uplink directions. Furthermore, all repeater systems aredesigned to preserve or restore balance in or to the mobiletelecommunication system. This is so ingrained into designs that therecan be said to be a “culture of balance” in which everyone accepts it asa given that they must achieve and maintain a balanced system.

Besides CDMA cellular systems, non-cellular systems, such as WiFi 802.11systems and WiMax 802.16 systems, also suffer from co-channelinterference in high-rise and other environments because of a lowfrequency reuse factor.

It is against this background and with a desire to improve on the priorart that the present invention has been developed.

SUMMARY

One aspect relates to a repeater for a 2-way mobile communication systemhaving a plurality of base stations and subscriber units thatcommunicate via RF signals includes a base-side antenna that is directedtoward at least one of the base stations to receive transmitted RFsignals from the base station, the antenna generating a receiveddownlink signal therefrom. The repeater also includes an amplifierreceptive of the received downlink signal, the amplifier amplifying thelevel of the received downlink signal to generate an amplified downlinksignal. The receiver further includes a subscriber-side antenna locatedproximate to the base-side antenna, the subscriber-side antenna beingreceptive of the amplified downlink signal, the subscriber-side antennatransmitting RF signals representative of the amplified downlink signalwithin a local area to subscriber units located nearby. The repeateroperates in the downlink direction between the base station and thesubscriber units near the repeater.

The repeater further includes amplification in the uplink directionwhich is substantially less than the amplification used in the downlink.The amplification in the uplink direction includes a subscriber-sideantenna receiving RF signals within a local area from subscriber unitslocated nearby, the antenna generating an uplink received signaltherefrom; an amplifier receptive of the received uplink signal, theamplifier amplifying the received uplink signal by a substantiallysmaller level then used in the downlink, and which generates anamplified uplink signal; and a base-side antenna that is directed towardat least one of the base stations, the subscriber-side antenna beingreceptive of the amplified uplink signal, and which transmits RF signalsrepresentative of the amplified uplink signal to the base station. Therepeater operates in the uplink direction between the subscriber and thebase station units near the repeater and the presence of the repeater inthe downlink and uplink directions causes an intentional imbalance intotal losses between the downlink and the uplink, in order to providefor signal enhancement in both uplink and downlink over an equivalentcoverage area.

The RF signals received by the base-side antenna and the RF signalstransmitted by the subscriber-side antenna may be at substantially thesame frequency, which is different from the frequency of the RF signalsreceived by the subscriber-side antenna and the RF signals transmittedby the base-side antenna. The repeater may further include a repeaterhousing in which the subscriber-side and base-side antennas are located.The base-side and subscriber-side antennas may be located within twometers of each other.

The amplifier may include circuitry therein to substantially preventsaturation. The circuitry may substantially prevent saturation byperiodically incrementing the magnitude of amplification toward anoscillation condition and testing for same. The circuitry may reset themagnitude of amplification to a smaller level during periodic testingfor oscillation, and to a much smaller level for sudden occurrences ofoscillation, in order to minimize the length of time for which steadyamplification is interrupted. The period with which the circuitry resetsthe magnitude of amplification may be gradually increased from arelatively shorter duration toward a relatively longer duration ifsuccessive tests for oscillation indicate a steady and adequateamplification setting, and wherein the reset period may be decreasedwhen oscillation is detected.

The base-side and subscriber-side antennas of the repeater may bepositionable by the operator for signal enhancement. The amplifier mayprovide an external indication of the magnitude of amplification andwherein the subscriber uses the external indication to optimallyposition the base-side and subscriber-side antennas of the repeater. Therepeater may be effective in substantially eliminating co-channelinterference in the vicinity of the repeater, such as within ten metersof the repeater. The subscriber units and the base-side andsubscriber-side antennas of the repeater may all be located in thevicinity of each other and all are in an environment receiving signalsof adequate signal strength from multiple base stations.

The mobile communication system may include a downlink direction inwhich signals are transmitted from the base stations to the subscriberunits and an uplink direction in which signals are transmitted from thesubscriber units to the base stations and wherein the repeater system isemployed in both the downlink and uplink directions, creating anintentional imbalance in order to resolve co-channel interference whileminimizing the risk of interference to the same communication system orto other communication systems.

The base-side and subscriber-side antennas may be isolated from eachother using cross-polarization achieved by mechanical orientation or byelectrical rotation. The base-side and subscriber-side antennas may beisolated from each other using a metallic chassis. The difference inuplink and downlink amplification may be limited in magnitude tomaintain the capacity of the mobile communications system to supportlink imbalance in a 2-way system. The repeater may further include asecondary interior repeater for two-way amplification between the firstrepeater and the subscriber.

Another aspect relates to a repeater system for a 2-way mobilecommunication system having a plurality of base stations and subscriberunits that communicate via RF signals, including a first repeater andsecond repeater. The first repeater includes: a base-side antenna thatis directed toward at least one of the base stations to receivetransmitted RF signals from the base station, the antenna generating areceived downlink signal therefrom; an amplifier receptive of thereceived downlink signal, the amplifier amplifying the level of thereceived downlink signal to generate an amplified downlink signal; and asubscriber-side antenna located proximate to the base-side antenna, thesubscriber-side antenna being receptive of the amplified downlinksignal, the subscriber-side antenna transmitting RF signalsrepresentative of the amplified downlink signal within a local area. Thesecond repeater includes: a base-side antenna that is directed towardthe first repeater to receive transmitted RF signals from the firstrepeater, the antenna generating a received downlink signal therefrom;an amplifier receptive of the received downlink signal, the amplifieramplifying the level of the received downlink signal to generate anamplified downlink signal; and a subscriber-side antenna locatedproximate to the base-side antenna, the subscriber-side antenna beingreceptive of the amplified downlink signal, the subscriber-side antennatransmitting RF signals representative of the amplified downlink signalwithin a local area to subscriber units located nearby.

The second repeater operates in the downlink direction between the basestation and the local area. The second repeater further includesamplification in the uplink direction which is substantially less thanthe amplification used in the downlink. The subscriber-side antenna ofthe second repeater receives RF signals within a local area fromsubscriber units located nearby, the antenna generating an uplinkreceived signal therefrom. The amplifier of the second repeater isreceptive of the received uplink signal, the amplifier amplifying thereceived uplink signal by a substantially smaller level then used in thedownlink and generating an amplified uplink signal. The base-sideantenna of the second repeater is directed generally toward at least oneof the base stations, the base-side antenna being receptive of theamplified uplink signal and transmitting RF signals representative ofthe amplified uplink signal to the at least one of the base stations.The second repeater operates in the uplink direction between thesubscriber and the first repeater and the presence of the secondrepeater in the downlink and uplink directions causes an intentionalimbalance in total losses between the downlink and the uplink, in orderto provide for signal enhancement in both uplink and downlink over anequivalent coverage area.

Another aspect relates to a repeater for a mobile communication systemhaving a plurality of base stations and subscriber units thatcommunicate via RF signals. The repeater includes a base-side antennathat is directed toward one of the base stations to receive transmittedRF signals from the base station, the base-side antenna generating areceived signal therefrom. The repeater also includes an amplifierreceptive of the received signal, the amplifier amplifying the level ofthe received signal to generate an amplified signal, the amplifierincluding circuitry therein to substantially prevent saturation byincrementing the magnitude of amplification toward an oscillationcondition and testing for same. The repeater further includes asubscriber-side antenna located proximate to the base-side antenna, thesubscriber-side antenna being receptive of the amplified signal, thesubscriber-side antenna transmitting RF signals within a local area tosubscriber units located nearby.

The repeater may further include a repeater housing in which thebase-side and subscriber-side antennas are located. The repeater mayfurther include an uplink amplifier of substantially less amplificationthan the downlink amplifier.

The mobile telecommunication system may include a downlink direction inwhich signals are transmitted from the base stations to the subscriberunits and an uplink direction in which signals are transmitted from thesubscriber units to the base stations, and the presence of the repeatermay cause an intentional imbalance in total losses between the downlinkand the uplink directions. The amplification may be limited in magnitudeto maintain the capacity of the mobile communication system to supportlink imbalance.

The repeater may be effective in substantially eliminating co-channelinterference in the vicinity of the repeater, such as within ten metersof the repeater. The subscriber units and the base-side andsubscriber-side antennas of the repeater may all be located in thevicinity of each other and may all be receiving signals of adequatesignal strength from multiple base stations.

The mobile telecommunication system may include a downlink direction inwhich signals are transmitted from the base stations to the subscriberunits and an uplink direction in which signals are transmitted from thesubscriber units to the base stations and the repeater system may beemployed in the downlink direction only.

The mobile telecommunication system may include a downlink direction inwhich signals are transmitted from the base stations to the subscriberunits and an uplink direction in which signals are transmitted from thesubscriber units to the base stations and the repeater system may beemployed in both the downlink and uplink directions, creating anintentional imbalance in order to provide enhanced subscriber servicewhile minimizing the risk of interference to the same communicationsystem or to other communication systems.

The saturation-prevention circuitry may reset the magnitude ofamplification to a smaller level during periodic testing foroscillation, and to a much smaller level for sudden occurrences ofoscillation, in order to minimize the length of time for which steadyamplification is interrupted.

The elapsed time after which the circuitry resets the magnitude ofamplification may be gradually increased from a relatively shorterduration toward a relatively longer duration if successive tests foroscillation indicate a steady and adequate amplification setting, andthe reset period may be decreased when oscillation is detected.

Another aspect relates to a method for substantially eliminatingco-channel interference in a local area containing subscriber unitsreceiving adequate signals from a plurality of base stations. The methodincludes co-locating a subscriber-side antenna and a base-side antennawithin an environment receiving signals of adequate signal strength frommultiple base stations to provide downlink amplification andsubstantially lower uplink amplification. The lower level of uplinkamplification substantially reduces the risk of creating interference tothe same communication system or to other communication systems.

Link imbalance may be intentionally caused between the downlink and theuplink directions, said link imbalance being less than thetelecommunication system's capacity for tolerating link imbalance. Themethod may further include substantially preventing saturation byperiodically testing for oscillation with a minimum of serviceinterruption and then setting the amplification to a maximum magnitude.

Numerous additional features and advantages of the present inventionwill become apparent to those skilled in the art upon consideration ofthe further description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the setting in which the repeater system ofthe present invention can be used.

FIG. 2 is an illustration of a portion of the setting shown in FIG. 1,showing an exterior room in a building.

FIG. 3 is an illustration of repeater system of the present invention ina typical setting.

FIG. 4 is a block diagram of the repeater system of the presentinvention containing downlink-only amplification.

FIG. 5 is a block diagram of an algorithm of the present invention foroptimally setting-up the repeater system of the present invention.

FIG. 6 is an illustration of typical RF energy patterns associated withthe repeater system of the present invention.

FIG. 7 is a map of approximate values of RF energy associated with therepeater system of the present invention.

FIG. 8 is a block diagram of an algorithm of the present invention forpreventing saturation with the repeater system of the present invention.

FIG. 9 is a plot of the amplification gain of the repeater system of thepresent invention during various stages of gain, described by thealgorithm shown in FIG. 8.

FIG. 10 is an illustration of the signals sent and received by and froma base station and a subscriber unit when the repeater system of thepresent invention is employed.

FIG. 11 is an illustration of a portion of the setting shown in FIG. 1,showing an exterior and interior room of a building.

FIG. 12 is a block diagram of the repeater system of the presentinvention containing weak uplink amplification.

FIG. 13 is a plot of the amplification gain of the repeater system ofthe present invention during various stages of dwell time, described bythe algorithm shown in FIG. 8

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made to the accompanying drawings, which assist inillustrating the various pertinent features of the present invention.Although the present invention will now be described primarily inconjunction with solving pilot pollution problems in high-risebuildings, it should be expressly understood that the present inventionmay be applicable to other applications where a solution to co-channelinterference in strong signal environments is required/desired. In thisregard, the following description of a system that solves pilotpollution problems in high-rise buildings is presented for purposes ofillustration and description. Furthermore, the description is notintended to limit the invention to the form disclosed herein.Consequently, variations and modifications commensurate with thefollowing teachings, and skill and knowledge of the relevant art, arewithin the scope of the present invention. The embodiments describedherein are further intended to explain modes known of practicing theinvention and to enable others skilled in the art to utilize theinvention in such, or other embodiments and with various modificationsrequired by the particular application(s) or use(s) of the presentinvention.

A repeater system 20 of the present invention is shown in FIG. 1. As canbe seen, the repeater system 20 is located in a high-rise building 22,in the interior of an external room 24 in the building, preferably neara window 26. As will be the case in the vicinity of most any high-risebuilding in the world, a mobile or cellular telephone system 30 existsin the surrounding area of the building 22. Located within or nearby theroom 24 in the building 22 may be one or more subscriber units 32(cellular or other wireless telephones).

The mobile telephone system 30 includes a plurality of base stations 34located in the vicinity of the building 22. As is well known, each ofthese base stations 34 may operate at different transmit and receivefrequencies than adjacent base stations 34 for TDMA and GSM technologysystems, while they may operate at the same transmit and receivefrequencies as adjacent base stations for CDMA technology systems. Whilethe present invention is not limited to CDMA systems, the embodimentsdescribed herein all may refer to CDMA systems, purely for convenience.The present invention applies to any spread spectrum communicationssystem, and to any communications system with a frequency reuse factorthat results in debilitating co-channel interference in some locations.Each base station 34 has an antenna system 36 associated therewith.

As shown in FIG. 2, there may be several subscribers carrying subscriberunits 32 in the room 24 having a window 26. It can be seen that thereare many competing downlink signals 40 that would be potentiallyreceived by the subscriber units 32 were it not for the presence of therepeater system 20. The repeater system 20 receives selected ones ofthese downlink signals 40 via a base-side antenna 200, amplifies themwith an amplifier 300, and transmits an amplified downlink signal 42from a subscriber-side antenna 400. Stability control circuitry 384maintains maximum gain as the isolation between antennas 200 and 400varies. Due to the relatively greater signal strength of the amplifieddownlink signal 42 in the vicinity of the repeater system 20, each ofthe subscriber units 32 use that signal 42 rather than the competingdownlink signals 40. The subscriber units 32 send uplink signals 44directly back to the base station in conventional fashion. There isgenerally no need for a repeater system for the uplink direction,although one could be employed to keep link imbalance within thesystem's capacity to support link imbalance, or if greater service rangeis desired.

FIG. 3 illustrates CDMA co-channel interference signals and theoperation of an interference repeater in a windowed room. As shown inFIG. 3, a two-way cellular network 100 blankets the user area with manystrong signals. The user area, generally an indoor office or apartment,resides in a strong signal environment (greater than −90 dBm), common tohigh-rise urban locations, and is receiving same-frequency signals fromtwo or more base stations of the desired communication system(provider). Cellular network 100 is composed of cell sites (basestations), telephony switches, backhaul, and all other elementsnecessary to create a voice and data infrastructure. Network 100provides wireless access to cellular subscriber units 500. Candidatedonor cell sites 101 each provide a strong line-of-sight (LOS)communications downlink signal 111 to the subscriber and downlink signal110 to the base-side antenna 200 of the repeater 20. Signals 110 and 111may be approximately equal to each other, and uniform over the region ofstrong or adequate signal strength from multiple signals, which may becalled the volume of interference 150. Often, these candidate donorcells can be visually seen through a window in the user area, and mostof the energy contained in the LOS signals 110 and 111 lie within arelatively narrow range of azimuth. This may allow the base-side antenna200 to be a moderately directional antenna of beamwidth <180°, possiblyin the range of 60° to 120°. Once signals 111 penetrate into a metallicenvironment, like a windowed office, reflection and diffraction on edgesand surfaces spread the energy over a wider angle approaching 360°,reducing the fraction of available energy that can be collected from adirectional antenna.

The complement to downlink signal 111 is uplink signal 511, whichpossesses approximately the same path loss after averaging outsmall-scale signal fading. A balanced path exists when the downlink pathloss, defined as the difference between donor cell transmit power andsubscriber received power, equals the uplink path loss, defined as thedifference between subscriber transmit power and donor cell receivedpower.

Interference cell sites 102 provide strong non-line-of-sightcommunications signals 112 to subscriber unit with much scattering byobjects in the environment. These signals arrive over a wide range ofazimuth angles approaching 360 degrees. For instance, these signals 112may bounce off other buildings and come through side walls beforearriving in the user area. In general, signals from interference cells102 (non-LOS) are not collectable, and do not represent qualified donorcells.

As one moves further into a scattering environment, such as a building,it becomes increasingly difficult to select specific donor cells using adirectional antenna. Therefore, the best place to collect LOS ornear-LOS signals may be at the edge of a scattering environment, such asnear a window, pointed away from scattering objects.

Co-channel interference occurs when two or more AMPS, TDMA, or GSMsignals of the same frequency are received by the subscriber within thevolume of interference 150, at the same time, and at comparable signallevels (such as signals 111 and 112). For CDMA, interference can occurwhen four or more signals of the same channel are received in the userarea at comparable signal levels, and the interference is called pilotpollution. A beacon signal called the pilot is always transmitted fromeach CDMA cell site, which subscribers select and receive in order to beready for a call. When the subscriber unit is on but not in a call,co-channel interference (or pilot pollution) prevents a cellular callfrom initiating, or at least reduces the likelihood of a successfulinitiation (call set-up). When a two-way call is in progress, co-channelinterference degrades call quality as signal conditions vary (whichoccurs as the subscriber unit wanders within the user area), and maycause the call to drop completely.

The base-side antenna 200 (FIGS. 3 and 4) selects signals 110 from oneto three candidate donor cells 101. The base-side antenna 200 may have ahorizontal beamwidth 208 (FIG. 6) sufficiently wide enough to collectmost of the power from each line-of-sight candidate cell 101, andsufficiently narrow to reject all but one to three candidate donorcells, and pointed typically out a window, converts the signal 110 intoan electrical signal 301 (FIG. 4). All other candidate donor cells andmost interference cells are rejected by the narrow beamwidth 208 of thebase-side antenna 200. Since the repeater 20 is a short distance(typically less than ten meters) from the subscriber unit, the signal110 received by the repeater 20 is approximately equal in strength tosignal 111 available at the subscriber unit directly from the donor cell101.

The antenna 200 may include a patch antenna 201 and an optional pivot203 (FIG. 4) that allows the installer to point the base-side antenna200 in different directions in order to select one to three strong donorcells. To install the repeater, the installer or subscriber iterates themounting location of repeater 20 and pointing angle of the patch antenna201 while monitoring downlink audio quality and an amplificationindicator 384, until maximum amplification is obtained and audio qualityis improved. Further detail on this iterative process is available inFIG. 5 and the accompanying description.

As shown in FIG. 11, two repeaters could be used to allow subscribers toutilize their subscriber units in an interior room. A first repeater 20provides downlink-only amplification of selected signals 40 out to adistance limit 27. In actuality, the distance limit 27 is not nearly assharply- or well-defined as illustrated. A secondary interior repeater21 extends 2-way signal enhancement to the interior 25 of the building.Secondary interior repeater 21 has an uplink amplifier 303 whose gainand rated power output is substantially less than the gain of a downlinkamplifier 302, and where the uplink gain compensates for additional linkimbalance which occurs due to repeater 20, and/or compensates foropposite signal fading which is greater in the building interior than atthe window, and/or which compensates for additional signal lossesbetween distance limit 27 and window 26. The uplink gain is ofsubstantially lower saturated power and lower gain, having just enoughgain to maintain a limited link imbalance for subscribers close toantenna 410. There may be several subscribers carrying subscriber units33 in interior room 25. Competing downlink signals 40 and weak amplifieddownlink signals 42 penetrate into room 25 and they would be potentiallyreceived by the subscriber units 33 were it not for the presence of therepeater system 21. The repeater 21 receives downlink signal 42 fromrepeater 20 via an antenna 210, amplifies them with a downlink amplifier302, and transmits a secondarily-amplified downlink signal 43 from anantenna 410. Stability control circuitry 385 maintains maximum gain asthe isolation between antennas 210 and 410 varies. Due to the relativelygreater signal strength of the secondarily-amplified downlink signal 43in the vicinity of the repeater 21, each of the subscriber units 33 usethat signal 43 rather than the competing downlink signals 40 or weakamplified downlink signals 42. The subscriber units 33 send uplinksignals 45 to antenna 410, which are amplified by uplink amplifier 302,and transmit amplified uplink signal 46 directly back to the basestation via antenna 210.

One procedure for installing and optimizing a primary window repeater 20is shown in FIG. 5. The same procedure can be extended to a secondaryinterior repeater 21, where a ceiling mount is chosen instead of awindow, and where the primary repeater is the indirect source of a donorsignal instead of donor base stations directly.

Refer to FIGS. 4, 5, 11, and 12 for the following description. First,repeater 20 is turned on (step 80), and located (82) near or on awindow. Optimally, suction cups are used to quickly attach the repeaterto a window facing into the user's office or apartment. Alternatively,the repeater can be mounted on a table fixture or inside a lamp forpositioning within 1 m of a window. Next, repeater 20, containingantennas 200 and 400, is pointed (84) towards line-of-sight donor cells101. Standing at one to two meters distance from and in front of thesubscriber-side antenna 400, having established a call, the usermonitors (86) the received audio quality of subscriber unit 32 formuting and audio signal break-up, and adjusts the pointing angle ofbase-side antenna 200 until minimal audio muting or degradation occurs.(The repeater is optimized to work well at or beyond some minimumoperating distance naturally encountered in the subscriber's environment(e.g. 1 m). At this minimum operating distance, link imbalance will bejust within the system's capacity to support imbalance, so it isimportant to not stand within this minimum distance duringinstallation.) If the tested audio quality is inadequate or not improved(NO), another pointing direction is chosen (84) and the audio qualityretested (86). Generally, given a 60-90 degree horizontal beamwidth 208(FIG. 6), only 3 possible pointing directions need be evaluated and oneor more of those will work well.

Once one or more pointing directions are found to yield good audioquality at 1-2 meters distance (YES), the user observes (88) the gainindicator 384 and optimizes the mounting location for maximum gain,maintaining the same pointing direction discovered in the previoussteps. In the preferred embodiment, the gain indicator may indicate low,medium, or high gain, each being 5 dB different in level. If the gainindicator is not at maximum (NO), there may be too much coupling betweenantennas 200 and 400 due to nearby metal, window framing, metallizedtint, or other room reflections. The user then relocates (98) themounting location and observes the gain, iterating to get the bestpossible gain indication, averaged as the user moves about the room.

Once gain has been optimized (YES), the user moves about the room,listening to downlink audio quality received during an active call, andnotes (92) the usable operating distance of the device within and beyondthe area he or she desires to improve. If performance is adequate (YES),the installation is complete and any final mounting attachments can besecured. The user may adjust pivot 403 of subscriber-side antenna 400 inorder to illuminate more uniformly the room area over which call qualityimprovement is desired. If operating distance is not adequate (NO), thenthe gain indicator 384 is checked to see if displayed gain is at itsmaximum indication (94). If NO, then antenna 400 can be removed (96)from the unified housing in order to create more isolation betweenantennas 200 and 400, and a coaxial cable (not shown) can be insertedbetween the repeater and the subscriber-side antenna 400.

If gain is at maximum (step 96, YES result) but operating distance isinadequate, then it may be necessary to iterate base-side pointing steps84 and 86, and then re-optimize the mounting location using steps 88,90, and 98.

Repeater 21 is installed near the edge of coverage boundary 27 andpointed to a primary repeater 20. Then, the same steps as above are usedto establish a maximal area of improved service over area 25. Becausethere is additional uplink signal loss between repeaters 20 and 21,there may be excessive link imbalance at a 1 meter distance betweensubscriber 33 and repeater 21, reducing the ability to establish asuccessful communications link. It may be necessary to reduce the linkimbalance experienced by subscribers 33 at the closest natural operatingdistance by reducing the gain of repeater 21. Excess link imbalance canbe reduced by reducing downlink amplification using an adjustableexternal gain adjustment. In cases where repeater 21 has an uplinkamplifier, uplink gain can be increased until communications can bereliably established at the closest natural distance between subscriber33 and repeater 21.

Returning to one-way repeater 20, the amplifier 300 (FIG. 4) suitablyamplifies the received signal 110 for the downlink as follows.Electrical signal 301 passes through filter 310, which rejects signalsother than those desired for user communication which either could causeelectrical overload of the amplifier chain, or which, when amplified,could create interference for other users. Typically, this filter iswide enough to pass the entire PCS or cellular spectrum, or a licensedband within the PCS or cellular spectrum, and is narrow enough to rejectthe opposite link direction and other adjacent telecommunicationsservices.

An amplifier 320 increases the amplitude of the received signals 301.Band-select or channel-select stages 330, 333, 334, 336, and 338 may beincluded for additional filtering, beyond that of filter 310, in theevent more protection from, or less interference to, othertelecommunications users is needed. Amplified signal 321 is translatedto an intermediate frequency by mixer 330. IF stage 333 filters thetranslated signal 331 to obtain the desired interference protection.Mixer 338 translates the intermediate frequency signal back to theoriginal frequency of signal 321. Local oscillator (LO) 336 provides anun-modulated RF carrier to facilitate mixers 330 and 338.

Variable gain/attenuation stage 340 reduces the amplified signal 321 tokeep the overall amplifier chain from causing excessive power output dueto oscillation or excessively strong input signals. Oscillation canoccur if receive antenna 200 and transmit antenna 400 are notelectrically isolated by an amount greater than the amplification of theamplifier chain. FIGS. 6 and 7 provide some illustration of RF signallevels in the vicinity of the repeater system and the subscriber unit32. Intermediate amplifier 334 sets the signal level entering mixer 338for optimum linearity. A saturation protection circuit 380 controls thevariable gain/attenuation stage 340 via a V_(etl) (control voltage)signal 381, and controls an amplification indicator 384.

Amplifier 350 provides final amplification to signal 301 necessary toachieve the desired and stable amplification for the amplifier chain.The amplified signal 351 is measured by detector 360 to determine ifoutput power exceeds a preset threshold approaching saturation,indicating oscillation or excessively strong input signals. Filter 370performs final filtering to prevent interference to othertelecommunications users and systems.

Amplifier 300 limits the maximum amplification to a level which permitsreliable communications within the capacity of the system to supportimbalanced operation. In the case when the repeater contains an uplinkamplifier chain, the difference in uplink and downlink gain is limitedto a level that permits reliable communications within the capacity ofthe system to support imbalanced operation.

FIG. 12 shows simplified amplifier chains for the repeater 21 withdownlink gain and a substantially weaker uplink gain, this time as theonly repeater rather than as a secondary interior repeater. Although,shown as the only repeater in the system, this two-way repeater 21 couldalso be used as a secondary interior repeater or in any other suitablearrangement. Further, for the case of two repeaters, they could both betwo-way, they could both be one-way or there could be one of each asdescribed above for FIG. 11. Band-select or channel-select stages, suchas 330, 332, 333, 334, 336, and 338 described in FIG. 4, may be includedin order to enhance performance, if desired. For the downlink repeaterchain, base-side signal 110 is received by antenna 210 and filtered byduplexer 311. Variable gain stages 341 amplify the filtered signal tocreate a signal that is detected by detector 361 and filtered byduplexer 371, supplying amplified donor signal 42 radiated bysubscriber-side antenna 410 to the subscriber unit. Amplified donorsignal 42 is dominant over interference signals within the volume ofinterference 150. Typically, downlink amplification will be 45-50 dB. Inorder to maintain link imbalance within the capacity of the system tosupport link imbalance, an uplink amplifier chain may be used (this isparticularly the case if the repeater 21 is positioned in a buildingcore area and using a primary repeater as its donor cell, as shown inFIG. 11). Subscriber signals 44 are transmitted from the subscriber unit32 and received by subscriber-side antenna 410 where they are filteredby duplexer 371. Collected uplink signals are then amplified by variablegain stage 342. Amplified uplink signals are passed to duplexer 311 andprovided to base-side antenna 210 for transmission back to the source ofthe donor cells. Typically, uplink amplification can be 10-20 dB lessthan downlink gain and still maintain reliable cellular service for thesubscriber.

In a weaker-uplink embodiment of the invention, uplink amplification issubstantially less than downlink amplification (6 dB or more difference)in order to minimize the potential for interference to same andadjacent-spectrum cellular systems.

Saturation-prevention circuit and algorithm 380 maximizes theamplification while minimizing gain interruptions to the user andminimizing the occurrence of oscillation in user and adjacent spectra.In the following description, the term gain will be used, instead ofamplification, for briefness. Since the base-side antenna 200 andsubscriber-side antenna 400 (or 210 and 410) are co-located, theelectrical isolation may not be sufficient to allow maximum availablegain from the amplifier chain without oscillation. Electrical isolationwill vary over time as the user moves about the user area, and dependsgreatly on the size and construction of the room in which the repeateris located. An automatic saturation-prevention circuit and algorithm istherefore needed to test for oscillation and to set the gain at justbelow that which causes oscillation, as shown in FIG. 8, in a way thatminimizes interruption to the call in progress. A plot of the gain ofthe repeater system 20 during various stages of this algorithm is shownin FIGS. 9 and 13. FIG. 8 describes the algorithm for maximizing gainwhile avoiding instability; it contains an adaptive level feature and anadaptive period feature. FIG. 9 gives an example of periodicallyresetting the gain wherein the level of reduction in gain at the startof a reset cycle adapts to the cause of the reset (adaptive level). FIG.13 gives three examples of resetting the gain wherein the period ofresetting the gain adapts to the gain history (adaptive period oradaptive dwell).

Refer to FIGS. 8, 9, and 13. Upon turning on (50) the repeater 20, gainis set (52) to a low level where stability is guaranteed (e.g. 35 dB inFIG. 13), even if there is very low isolation between antennas 200 and400 (FIG. 4). Now it is necessary to acquire a stable operating pointfor the repeater using algorithm blocks (54) and (60).

To establish a stable operating point for the succeeding dwell time(250), detector voltage 382 is measured (54) for indications ofsaturation during monitor interval 266 (typically 10 μS). The saturationthreshold, indicative of possible oscillation, will typically be severaldB below the actual 1 dB compression point of the final simplifier stage350 (FIG. 4) or 341 (FIG. 12) in order to ensure reliable detection ofpossible oscillation. For example, a final stage power rating of 20 dBmmight have a saturation detection threshold of 15 dBm. If there is nooscillation detected during the monitor interval 266, and gain has notreached the maximum gain, then gain is incremented (60) by step 252(typically 1-3 dB). Maximum gain (e.g. 55 dB) is the rated gain 270(e.g. 50 dB) of the repeater plus the stability margin 264 (e.g. 5 dB).Steps (60) and (54) continue until either oscillation is detected, oruntil maximum gain is reached. The number of incrementing steps will belarger following Turn On (e.g. 6 gain steps) than for periodic resettingbecause the gain must step up from a minimum (e.g. 35 dB) to the dwelllevel, say, 47 dB, in 2 dB steps, whereas periodic resets involve onlyone or two steps down from the next gain setting. Once maximum gain oroscillation occurs, algorithm reduces (56) the gain by stability margin264 to provide adequate margin for oscillation, typically 2-8 dB. Thisbegins the dwell time 250. After turning on the unit, the dwell timewill be set to the shortest duration since the environment couplingantennas 200 and 400 is unknown and may be rapidly changing.

The sequence of incrementing gain steps are referred to as gain“probes”, represented by label 268 in FIG. 9, and take up a relativelyshort amount of time (e.g. 20 μsec) compared to the dwell time 250 (e.g.1 sec). Therefore, dwell time 250 is approximately equal to the resetperiod 256. The new gain level is indicated (68) to the user byindicator 384 as an aid in picking a mounting location that gives thehighest isolation between antennas 200 and 400. Gain is also stored (74)in memory, and accumulated (76) over many reset cycles, in order toinform an adaptive period algorithm. The preceding paragraph describeshow a stable operating point is reached for one period of dwell. Now wewill monitor for oscillation during dwell, and describe how thealgorithm adapts its gain level to its environment.

As already described, detector voltage 382 is monitored (58) duringdwell for the unlikely occurrence of oscillation. Dwell is set so that,for the typical user, the algorithm tracks changes in the isolationbetween antennas 200 and 400 quickly enough that the periodic resettingof gain creates most of the incidences of oscillation. In that way, ashort period of resetting the gain allows decrement 260 to be kept toone or two gain steps so that the amount of time for which gain isreduced is minimal, thus minimizing the length of time during whichaudio quality may be interrupted. But, a longer period of resetting thegain, although increasing the rate of sudden occurrences of oscillationduring dwell, may reduce the net frequency with which saturation occurs,which minimizes the amount of interference that might be caused toadjacent spectrum subscribers. So there is a tradeoff between the numberof occurrences of audio interruption to the user and the number ofoccurrences of potential interference to nearby communications devicesin adjacent spectra.

The reset period is of the shortest duration (256, in FIG. 9), typically1 second, following turn on, or when the gain is less than the minimumgain for adequate coverage 258 (FIG. 13). As gain exceeds gain 258, andif successive reset cycles result in the same gain level as previouscycles, the reset period is increased in order to minimize gaininterruptions and occurrences of oscillation, eventually reaching alongest duration 257 (e.g. 100 seconds).

If oscillation is detected (58) during dwell, an emergency reset (62)occurs, whereupon gain is reduced by a large decrement 262, or all theway to the gain for guaranteed stability 254. Generally, large decrement262 has the effect of reducing gain to stable gain 254, so there is nodistinction between the two choices. Once emergency reset (62) occursand gain is reduced, the reset period is also reduced, usually to theshortest duration 256 (e.g. 1 second). After emergency reset (62), probesequences begin again (54, 60, 56) in order to establish a new stableoperating point.

If oscillation is not detected (58), the circuit can considerlengthening the reset period. First, a test is performed (64) of whetherthe gain meets a minimum for adequate coverage 258, meaning the typicalsubscriber experiences an adequate service range. For a personalrepeater, adequate gain might be 44 dB and yield a range of 3-5 meters,enough for a typical office. Although as much as 10 meters of rangemight be expected in an optimum deployment, it is advantageous to reducethe rate of service interruptions that potentially occur due toresetting of the gain. If the gain meets (64) level 258 (YES), the gainhistory is evaluated (70) to see if gain has been substantially constantfor several or many cycles of reset periods. If the gain has not beenparticularly constant (NO), then a periodic reset (66) occurs, wherebycontrol voltage 381 reduces the amplifier gain by small decrement 260(usually one or two steps of 2 dB each), and an iterative probe sequencebegins again in order to refresh the gain setting through steps 54, 60,and 56. If that gain in step 70 has been constant (YES), then the dwell(reset) period is incremented (72) in order to reduce the rate ofservice interruptions to the user without incurring a substantialincrease in sudden oscillation. Successive experiences of gain constancywill gradually lengthen the reset period to a longest duration (e.g. 100sec), which will then be retained until the isolation between antennas200 and 400 drops enough to cause a periodic or emergency resetting ofthe gain, wherein a shorter reset period will be introduced. Going backto step 64, if the minimum gain for adequate coverage (258) is not met(NO), then periodic reset 66 decrements gain by small step 260,following which the gain setting refreshed through iterative steps 54,60, and 56.

In FIG. 13, three examples of the time behavior of the adaptive periodportion of the algorithm are given. All three cases start with Turn On,incrementing the gain towards a stable and maximum gain setting affordedby the antenna isolation available in the user setting. Arrows at thetop of the graph show where some of the saturation occurrences(oscillation) happen due to periodic gain probes. In Case 2, a fat arrowindicates a “sudden” occurrence of oscillation during dwell and notprovoked by gain probes.

Case 1 illustrates a high-isolation environment with little or no usermovement near the repeater, allowing a high gain setting. Since there islittle opportunity for higher gain, it is best to forgo resetting thegain and receive the benefits of no gain interruptions and no potentialinterference from brief oscillation.

Case 2 illustrate a medium gain situation with some user movement.Initially, the reset period is shortest 256 since there is no history ofenvironmental stability. Then, eventually, gain constancy allows thedwell (reset period) to lengthen to 100 seconds, for example, untileventually a reduction in the isolation of the environment causes thedetection of instability in step 58, leading to an emergency reset 62and a decrementing of the reset period to 1 second. One could alsoimplement a gradual reduction in dwell through an intermediate durationinstead of dropping immediately to the shortest dwell.

Case 3 illustrates either a medium-to-high isolation environment with atemporary reduction in isolation due to user movement, or alow-isolation environment. In this case, gain is below the minimum gainfor adequate coverage for the typical subscriber and so it is preferableto seek higher gain at a fast pace so that the user will experienceadequate gain for the largest possible percentage of time. Therefore,the reset period 256 is at the shortest (e.g. 1 second). Note that theoptimum rate for shortest and longest reset period may be a factor of 10or more different than given in this example, depending on theapplication.

Referring to FIG. 6, subscriber-side antenna 400 illuminates the userarea, and is isolated from base-side antenna 200. Amplified donor cellsare transmitted into the user's area, creating a dominant set of signalsfrom donor cells and overtaking the interference cells. Antennas 200 and400 are isolated to maximize allowable amplification. Antenna isolationcan achieved through cross-polarization between 200 and 400, by metallicshielding of chassis 305, through optimizing the position of thechassis, and/or by small physical separation within the volume ofinterference 150. The amplification indicator 384 displays the amount ofamplification so the user can pick the mounting location for highestisolation and maximum allowable amplification. Amplification indicator384 may be a visible indicator such as an LED display, an LCD display,or other suitable type of visible indicator, or it may be an audiosignal whose frequency changes in proportion to amplification, or othersuitable audible indicator.

The amplified donor cell signal, being an appropriately-amplifiedversion of selected donor cell signal 110, passes to subscriber-sideantenna 400, which is pointed into the user area, usually an office orapartment. The beamwidth and pointing direction of antenna 400 is chosento maximally illuminate a volume of recovered service 155. The repeaterprovides an amplified version of the dominant donor signal(s) 110 a atthe subscriber unit and within the user area.

Referring to FIGS. 6 and 7, the donor signal gain 115 is equal to thedifference in the power between amplified signal 110 a and donor signal110. Amplified signal 110 a may dominate over the interference signals112 by at least several dB at the edge of the recovered volume 155, andby typically 20 dB at 1 m. The signal gain experienced by the subscribercorresponds to the magnitude of the imbalance introduced by therepeater. Antennas 200 and 400 are co-located in the same room,generally in the same mechanical package, and also isolated electricallyto allow maximum possible amplification. A back-to-back orientation,which points the lowest-amplification portions of each antennas pattern(204, 404) toward each other, creates typically 30 dB of isolation (FIG.6). Repeater chassis 305 is metallic and provides additional electricalisolation between antennas 200 and 400. The polarization of antennas 200and 400 may be set to 90° to acquire an additional 5-30 dB of isolation.Subscriber-side antenna 400 can be detached from chassis 305 andseparated by several feet in order to increase isolation further.Additionally, techniques to electrically rotate the polarization of oneantenna, with respect to the other antenna, may be employed,automatically optimized by saturation prevention circuit 380.

Antennas 200 and 400 may be separated (not mounted to a commonstructure) as long as the conditions for antenna and signal co-locationare maintained. Co-location exists if both antennas lie within thevolume of interference common to the subscriber unit, and as long asboth antennas are approximately in the same plane (e.g. a generallyvertical plane), such that the signal gain 115 is the same as when theantennas are physically co-located. Because donor signal strength isapproximately uniform across a small area, such as a windowed room,collocating the antennas establishes a fixed signal gain, given amaximum amplification, over the donor signal that the subscriber wouldreceive without a repeater. Co-location and limited amplification, then,create a repeater system which operates within the capacity of thesystem to support imbalanced downlink operation. Since the donor signalsare LOS or near-LOS, downlink signal fading is modest or minimal,freeing up a reserve capacity for the system to support imbalance whichcan be applied to the repeater system. Additionally, uplink spacediversity, provided by 2 or more base station antennas, is preservedsince there is no uplink amplification between the subscriber and thedonor cell. Therefore, composite uplink small-scale signal fading isminimal, whereas a conventional coverage repeater would eliminate uplinkdiversity, creating a downlink imbalance of its own.

When extending the invention with a second or further repeater into theinterior of a building, additional path losses occur between the donorsignal as received at the base-side antenna on the primary windowrepeater and the donor signal received at an interior subscriberlocation. In this case, a further imbalance, perhaps 5-10 dB, willoccur, using up the limited capacity of a typical CDMA system (24 dB) tosupport link imbalance. Further, opposite fading, which is minor in thenear-LOS (line-of-sight) environment of the windowed room, willincrease, further imbalancing the links during some moments in time.Adding an uplink amplifier chain whose gain is typically 10-20 dB lessthan the downlink will keep the repeater system within the system'scapacity for link imbalance. Because signal strengths are typically −90to −50 dBm, a rated output power for the uplink can be as low as 0.1 mW,substantially limiting the risk of uplink interference to the cellularbase station receivers, compared to conventional coverage repeaters.

Referring to FIGS. 3 and 10, the power control of a subscriber call isdiscussed, managed mainly by the cellular network. Subscriber unit 500receives downlink signals 110 a via antenna 540 from a downlink-onlyrepeater 20 and transmits uplink signals 511 to the network viatransmitter 530 and antenna 540. Signal 110 a from the selected donorcell(s) is greater in amplitude than the ambient signals 110 and 111,eliminating co-channel interference for subscriber 500.

Without the repeater, and at the start of a cellular call, networksignal 111 is received by antenna 540 and then receiver 510. At anintermediate frequency of receiver 510, receive signal strength 525 ismeasured, which is used to determine the initial open-loop power controltransmit level. A digitized message containing signal strength 525 isalso relayed back to the network for subsequent tracking of subscriberbehavior. Prior to subscriber transmission there is initially nosubscriber signal quality to measure at network 100; so the subscribercalculates an open-loop power with which to transmit. Based on a formulawhich assumes a typical base station transmit power level (e.g. 5 watts)and assuming that uplink and downlink path losses are equal, the formulais “Transmit Power=−73-RSSI dBm”. RSSI is the received signal strengthindication 525. Subscriber transmitter 530 transmits an initial message,via antenna 540, back to network 100. Network antenna 106, receiver 130and transmitter 120 respond by setting up a call with subscriber 500.Once a call is set up, signal quality 132 begins to assess the uplinksignal quality, usually against a frame error rate (FER) target of 2-3%.At a rate of 800 times per second, closed-loop power control message 122is sent to the subscriber on downlink 111. Power control message 122 isdecoded by decoder 520, and transmitter 530 power level is adjusted upor down to keep received signal quality 132 at or near the targetprogrammed into network 100. Thus, the second power controlmechanism—closed loop power control—displaced the initial open-looppower control mechanism, and will continue to determine subscribertransmit power for the duration of the call.

Nominally, the power control setting determined by open-loop powercontrol and the power control setting determined by closed-loop powercontrol will be approximately the same without a repeater. Once a callis ongoing, network 100 continues to track received signal strength 525in the event the theoretical set point for open-loop power control woulddeviate substantially from the closed-loop setting. This deviation mightoccur in the event of equipment failure, which would then precipitate ashutting down of the call in order to protect the rest of the network.The maximum allowed deviation between open-loop and closed-loop powercontrol settings is typically 24 dB during a call and 32 dB at the startof a call, but can be programmed to be a different value by the networkoperator. One condition which offsets the open- and closed-loop powersettings is signal fading, which occurs independently on the uplink anddownlink of a frequency duplexed PCS system.

With a downlink-only repeater 20 activated, signal 110 a causes thesubscriber unit to report a stronger signal strength 525 than it wouldwithout a repeater, and therefore subscriber unit 500 appears to becloser to the cell site than it actually is. So upon initial call setup, the open-loop transmit power level of the subscriber is set,according to “Transmit Power=−73-RSSI (525) dBm, to a smaller powerlevel. Since repeater signal 110 a is stronger than the un-boosteddonor-cell signal 111 by the signal boost (typically 5-20 dB over one toten meters distance from the repeater), the initial subscriber transmitpower will be 5-20 dB lower than without a repeater. Fortunately, theCDMA system typically allows open-loop power to be offset by 32 dBduring call set up. At some level of repeater boost, call set-up will bedelayed or prevented. Thus, network 100 establishes an uplink powercontrol mechanism which is substantially independent of the downlinksignal, up to a limit of 24 dB, which permits a limited link imbalanceto exist for one or more subscriber units.

Referring to FIG. 7, contours of signal boost 115 decrease as thesubscriber moves away from the repeater 20, until finally the differencebetween signals 110 a and 111 approaches 0 dB, creating a smoothtransition to the remainder of the indoor environment. Thus, transfer ofthe subscriber call (hand off) is still possible for a subscriber servedby a downlink-only repeater.

As can be appreciated the present invention intentionally imbalances thedownlink and uplink directions by inserting a repeater into the downlinkdirection. In other words, the repeater system of the present inventionprovides non-zero differential gain in a local area. It has beendiscovered that it is the downlink direction that is most affected bypilot pollution. Providing a repeater in only one direction isadvantageous because it greatly reduces the cost of an individualsolution to co-channel interference, eliminates the need for skilledinstallation, and eliminates the need for spectrum oversight andrepeater monitoring for malfunction. Furthermore, the amount of signalgain is controlled to be within the mobile communication system'scapability to support imbalance. Also, the repeater system may work bestin areas of island coverage (e.g., where hand-offs are not needed).

Further, a secondary interior repeater can supplement the system,extending coverage into interior rooms. The secondary repeater receivesthe amplified downlink signal from the first repeater and amplifies it.It also receives the uplink signal from the subscriber unit, amplifiesit (by a lesser amount than the downlink signal), and sends theamplified uplink signal to the base station.

The foregoing description of the present invention has been presentedfor purposes of illustration and description. Furthermore, thedescription is not intended to limit the invention to the form disclosedherein. Consequently, variations and modifications commensurate with theabove teachings, and skill and knowledge of the relevant art, are withinthe scope of the present invention. The embodiments describedhereinabove are further intended to explain best modes known ofpracticing the invention and to enable others skilled in the art toutilize the invention in such, or other embodiments and with variousmodifications required by the particular application(s) or use(s) of thepresent invention. It is intended that the appended claims be construedto include alternative embodiments to the extent permitted by the priorart.

1. A repeater in a mobile communication system comprising a plurality ofbase stations and subscriber units that communicate via radio frequency(RF) signals, comprising: a base-side antenna configured to receive atleast one selected downlink signal from at least one of the plurality ofbase stations and destined for at least one of a plurality of subscriberunits and to generate a received downlink signal, the base-side antennaalso being configured to receive an amplified uplink signal and totransmit RF signals representative of the amplified uplink signal to oneof the plurality of base stations; a first amplifier, coupled to receivethe received downlink signal, and further configured to amplify a levelof the received downlink signal to generate an amplified downlinksignal; a second amplifier configured to receive and to amplify areceived uplink signal, transmitted by at least one of the plurality ofsubscriber stations and destined for at least one of the plurality ofbase stations, by a substantially smaller level then used in a downlinkdirection to generate the amplified uplink signal; and a subscriber-sideantenna, located proximate to the base-side antenna and coupled to thefirst amplifier to receive the amplified downlink signal, thesubscribe-side antenna configured to transmit RF signals representativeof the amplified downlink signal within a local area to nearbysubscriber units, and in an uplink direction, said subscriber-sideantenna being configured to receive RF signals from the nearbysubscriber units and to generate the received uplink signal.
 2. Therepeater as defined in claim 1, wherein downlink RF signals received bythe base-side antenna and downlink RF signals transmitted by thesubscriber-side antenna are at substantially a same frequency, thefrequency being different from a frequency of uplink RF signals receivedby the subscriber-side antenna and uplink RF signals transmitted by thebase-side antenna.
 3. The repeater as defined in claim 1, wherein eachof the base-side antenna and the subscriber-side antenna comprises apivot configured to point the base-side antenna and the subscriber-sideantenna in different directions.
 4. The repeater as defined in claim 1,further comprising control circuitry configured to maintain maximum gainas isolation between the base-side antenna and the subscriber-sideantenna varies.
 5. The repeater as defined in claim 1, wherein thesecond amplifier comprises: a first filter configured to filtersubscriber signals transmitted from the subscriber units; variable gainstages configured to amplify collected uplink signals; and a duplexerconfigured to provide amplified uplink signals to the base-side antenna.6. The repeater as defined in claim 1, wherein the first amplifiercomprises: a first filter configured to reject signals other thandesired signals for user communications; variable gain stages configuredto amplify a filtered signal; a detector configured to detect anamplified filtered signal; and a second filter configured to filter asignal detected by the detector and thereby supply an amplified signalto the subscriber-side antenna for transmitting to the subscriber units.7. The repeater as defined in claim 6, wherein the variable gain stagescomprise: a first sub-amplifier configured to increase an amplitude of areceived signal; a first mixer configured to translate an amplifiedsignal to an intermediate frequency; an intermediate frequency stageconfigured to filter a translated signal to obtain a desiredinterference protection; a second mixer configured to translate anintermediate frequency signal to an original frequency of the amplifiedsignal; and a local oscillator configured to provide an un-modulated RFcarrier to facilitate the first mixer and the second mixer.
 8. Therepeater as defined in claim 7, wherein the variable gain stages furthercomprise: an attenuation stage configured to reduce the amplified signaland to keep an amplifier chain from causing excessive port output due tooscillation or excessively strong input signals.
 9. The repeater asdefined in claim 6, wherein the first amplifier further includes asaturation protection circuit configured to control the variable gainstages via a control voltage signal and to control an amplificationindicator.
 10. The repeater as defined in claim 6, wherein the firstamplifier further comprises: a second sub-amplifier configured toprovide final amplification to the received signal to achieve a desiredand stable amplification, wherein the second sub-amplifier is configuredto supply an amplified signal to be measured by the detector todetermine if output power exceeds a preset threshold approachingsaturation, and wherein the second filter is configured to filter theamplified signal to prevent interference to other telecommunicationsusers and systems.
 11. The repeater as defined in claim 1, wherein therepeater is configured to substantially eliminate co-channelinterference in the vicinity of the repeater.
 12. The repeater asdefined in claim 1, wherein the repeater is configured to substantiallyeliminate co-channel interference within ten meters of the repeater. 13.The repeater as defined in claim 1, wherein the subscriber units, thebase-side antenna and subscriber-side antenna are located in thevicinity of each other and are in an environment to receive signals ofadequate signal strength from multiple base stations.
 14. The repeateras defined in claim 1, wherein the repeater system is employed in boththe downlink and uplink directions and is configured to create anintentional imbalance in order to resolve co-channel interference whileminimizing the risk of interference to the same communication system orto other communication systems.
 15. The repeater as defined in claim 1,wherein the base-side and subscriber-side antennas are isolated fromeach other using cross-polarization achieved by mechanical orientationor by electrical rotation.
 16. The repeater as defined in claim 1,wherein the base-side and subscriber-side antennas are isolated fromeach other using a metallic chassis.
 17. The repeater as defined inclaim 1, wherein the difference in uplink and downlink amplification islimited in magnitude to maintain the capacity of the mobilecommunications system to support link imbalance in a 2-way system. 18.The repeater as defined in claim 1, further including a secondaryinterior repeater for two-way amplification between the first repeaterand the subscriber.
 19. The repeater as defined in claim 1, wherein thepresence of the repeater in the downlink and uplink directions causes anintentional imbalance in total losses between the downlink and theuplink, in order to provide for signal enhancement in both uplink anddownlink over an equivalent coverage area.